Age of oceanic crust. Youngest crust(red) is along spreading centres.

Plates in the crust of the earth, according to the plate tectonics theory

Seafloor spreading occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. Download high resolution version (1624x1020, 895 KB)Age of oceanic crust. ... Download high resolution version (1624x1020, 895 KB)Age of oceanic crust. ... Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... A mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate tectonics. ... Age of oceanic crust Oceanic crust is the part of Earths lithosphere which underlies the ocean basins. ... For other uses, see Volcano (disambiguation). ... Plates in the crust of the earth, according to the plate tectonics theory Continental drift refers to the movement of the Earths continents relative to each other. ... The tectonic plates of the world were mapped in the second half of the 20th century. ...

Earlier theories (e.g., by Alfred Wegener) of continental drift were that continents "plowed" through the sea. The idea that the seafloor itself moves (and carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s. The theory is well-accepted now, and the phenomenon is known to be caused by convection currents in the plastic, very weak upper mantle, or asthenosphere. Alfred Wegeners theory of continental drift was widely ridiculed in his day Alfred Lothar Wegener (Berlin, November 1, 1880 – Greenland, November 2 or 3, 1930) was a German interdisciplinary scientist and meteorologist, who became famous for his theory of continental drift (Kontinentalverschiebung or die Verschiebung der Kontinente in his... Animated, colour-coded map showing the various continents. ... Harry Hammond Hess (1906-1969) was an American geologist. ... Princeton University is a private coeducational research university located in Princeton, New Jersey. ... The 1960s decade refers to the years from 1960 to 1969, inclusive. ... Convection in the most general terms refers to the internal movement of currents within fluids (i. ... This article does not cite any references or sources. ...

Incipient spreading

In the general case, sea floor spreading starts as a rift in a continental land mass, similar to the Red Sea-East Africa Rift System today. The process starts with heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to more dense objects, the area being heated becomes a broad dome (see isostasy). As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120 degree angles. These areas are named triple junctions and can be found in several places across the world today. It has been suggested that this article or section be merged with Rift valley. ... The Continental Crust is the layer of granitic and sedimentary rock which forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. ... Northern section of the Great Rift Valley. ... Isostasy is a term used in Geology to refer to the state of ice above stasy and is angravitational equilibrium between the Earths lithosphere and asthenosphere such that the tectonic plates float at an elevation which depends on their thickness and density. ... A triple junction is the point where three tectonic plates diverge. ...

Early spreading

If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift'. As the two active rifts continue to open, eventually the continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. The Red Sea is an example of a new arm of the sea. The East Africa rift was thought to be a "failed" arm that was opening somewhat more slowly than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment reported that in the Afar region last September, a 60 km fissure opened as wide as eight meters. During this period of initial flooding the new sea is sensitive to changes in climate and eustasy. As a result the new sea will evaporate (partially or completely) several times before the elevation of the rift valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley. Later these deposits have the potential to become hydrocarbon seals and are of particular interest to petroleum geologists. Map of Ethiopia highlighting the Afar region. ...

Sea floor spreading can stop at any time in the process, but if it continues far enough to the point that the continent is completely severed, then a new ocean basin is created. The Red Sea has not yet completely split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken completely free. South America once fit into the area of the Niger Delta. The Niger River has formed in the failed rift arm of the triple junction.

Continued spreading and subduction

The new oceanic crust is quite hot relative to old oceanic crust, so the new oceanic basin is shallower than older oceanic basins. Since the diameter of the earth remains relatively constant despite the production of new crust a mechanism must exist by which crust is also destroyed. The destruction of oceanic crust occurs at subduction zones where oceanic crust is forced under either continental crust or oceanic crust. Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge. Only a small portion of the oceanic crust produced in the Atlantic is subducted. However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Pacific Ring of Fire. The Pacific is also home to one of the world's most active spreading centres (the East Pacific Rise (EPR)) with spreading rates of up to 13 cm/yr. The Mid-Atlantic Ridge is a "textbook" slow spreading centre while the EPR is used as an example of fast spreading. The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced. The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone. ... “The Ring of Fire” redirects here. ...

Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean, sea level was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean. Western Interior Seaway during the mid-Cretaceous, about 100 million years before the present The Western Interior Seaway, also called the Cretaceous Seaway, the Niobraran Sea, and the North American Inland Sea, was a huge inland sea that split the continent of North America into two halves during most of...

Debate and search for mechanism

At the Mid-Atlantic Ridge (and other places), material from the upper mantle rises through the faults between oceanic plates to form new crust as the plates move away from each other, a phenomenon first observed as continental drift. When Alfred Wegener first presented a hypothesis of continental drift in 1912, conservative geologists, especially in North America, demanded to know where the motive force could possibly lie. Historically, the theory of seafloor spreading won nearly complete acceptance in the absence of evidence for the mechanism. Instead, the past record of geomagnetic reversals was noticed by observing the magnetic stripe "anomalies" on the ocean floor. This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from simply towing a magnetometer on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. The seafloor must have originated on the earth's great fiery welts, like the mid-Atlantic Ridge and the East Pacific Rise. Courtesy USGS The ridge was central in the breakup of Pangaea that began some 180 million years ago. ... Earth cutaway from core to exosphere. ... Earth cutaway from core to exosphere. ... Plates in the crust of the earth, according to the plate tectonics theory Continental drift refers to the movement of the Earths continents relative to each other. ... Alfred Wegeners theory of continental drift was widely ridiculed in his day Alfred Lothar Wegener (Berlin, November 1, 1880 – Greenland, November 2 or 3, 1930) was a German interdisciplinary scientist and meteorologist, who became famous for his theory of continental drift (Kontinentalverschiebung or die Verschiebung der Kontinente in his... Recent geomagnetic reversals. ... The East Pacific Rise is a long north-south welt of seafloor spreading under the eastern Pacific Ocean from near Antarctica in the south northward to its termination at the northern end of the Gulf of California in the Salton Sea basin in southern Pennsylvania California. ...

It is still a matter of some debate whether seafloor spreading is driven primarily by the force of rising magma at these locations, or if it is driven by the force of sinking oceanic crust at subduction zones and these upwellings are merely a side effect. It is likely however that some seafloor spreading is driven by active upwelling and some by passive upwelling. Magma is molten rock located beneath the surface of the Earth (or any other terrestrial planet), and which often collects in a magma chamber. ...

See also

• Divergent boundary
• Mid-ocean ridge

In plate tectonics, a divergent boundary (divergent fault boundary or divergent plate boundary), (but also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. ... A mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate tectonics. ...

External links

• Animation of a mid-ocean ridge